Bulk-storage bins, bunkers or silos, usually for flowable particulate, granular and like solids, can be provided with inwardly open helical or spiral chutes or ramps along an inner wall (or outwardly open chute along an outwardly facing wall) defining a bulk-storage compartment. The latter can be used in subterranean operations and is filled from the top. The mined or other particulate or broken mineral matter can be introduced at the upper end of the storage container by an inlet to the aforementioned chute and can be distributed thereby within the interior of this container.
Thus the chutes serve to discharge the bulk material introduced in the upper end of the chute more uniformly over the internal cross section of the bulk-storage container than systems which merely dump the solids into the bin from the upper end or top of the latter.
The chute, which is generally an inwardly open channel attached to or formed in the wall of the chamber, also facilitates total emptying of the bin.
The chute has the configuration of a screw thread, i.e. helix, which is inclined downwardly and can extend over the entire height of the bin in a plurality of turns. The chute thus has a tangential inclination such that, under the effect of gravity, the bulk material descends the chute so that its downward movement and momentum produces a centrifugal force which urges the material outwardly against the wall of the bin so that the chute can be opened inwardly, i.e. there is no need to fully enclose the path of the material along the chute.
In general, the chute has, in addition to this wall, against which the material is urged by centrifugal force, a ramp supporting the weight of the material against the vertical component of the gravitational influence. This ramp, which lies transverse to the vertical bin wall, is usually inclined downwardly and inwardly (hereinafter referred to as a "radial inclination") so that, in section in a vertical axial plane of the bin, the surface of this ramp is inclined to the horizontal. The combination of this tangential inclination and the radial inclination at any point or region of the helical chute gives rise to a "slope" or "gradient" of the ramp from the vertical wall of the bin to the free edge of the ramp lying inwardly of the wall. This slope or gradient is selected such that, upon emptying of the bin, any bulk material resting on the ramp will slide freely downwardly and hence accumulations of bulk material on the ramp are precluded. The slope is thus sufficient to permit the bulk material resting with zero momentum and solely its static inertia to begin to slide downwardly.
The bulk material is supplied to the helical chute by an inlet device which can include a laterally closed and either helical or straight inlet duct or chute. The inlet chute has its supporting wall or inlet ramp designed with zero radial inclination, i.e. in a vertical plane through this supporting wall, the wall is parallel to the horizontal and includes zero angle therewith.
Betweem this inlet chute and the helical chute, therefore, there is a transition region in which the horizontal ramp of the inlet chute merges with the radially inclined ramp of the helical chute. In this transition region, the radial inclination of the ramp continuously increases from zero (at its junction with the inlet chute) to the radial inclination of the helical chute at its junction therewith.
The bulk material introduced through the inlet chute and passed on to the helical chute is accelerated by its momentum at a rate determined by the tangential inclination. The acceleration becomes zero, i.e. the bulk material reaches a constant velocity, when the velocity-dependent component friction upon the bulk material is equal to the downwardly effective gravitational component along the helical chute.
The conventional bulk-material storage bins of the afore described type, the tangential inclination is constant over the entire length of the helical chute although the tangential inclination can be somewhat greater at the inlet chute. However, since the radial inclination of the transit region is practically zero at the start of this region, experience has shown that bulk material which accumulates in a static condition on the ramp surface is not discharged when the bin is emptied. To avoid this problem, the bin is filled only up to the top of the helical chute and the space above the helical chute, i.e. at the level of the transition region, remains empty. Consequently, a substantial portion of the storage bin remains unused and unusable in the conventional systems, thereby reducing the effective storage capacity of the unit.
While it might be suggested that one solution would be to increase the tangential inclination sufficiently to overcome this tendency of solid material to accumulate on the ramp surface of the transition region, this has not been found to be successful in practice because it detrimentally effects the velocity of the bulk material introduced to the chute and the movements of the material therealong.